Evidence that the contraction-induced rapid hyperemia in rabbit masseter muscle is based on a mechano-sensitive mechanism, not shared by cutaneous vascular beds.

Several mechanisms have been hypothesized to contribute to the rapid hyperemia at the onset of exercise. Aim of the present study was to investigate the role played by the mechano-sensitivity of the vascular network. In 12 anesthetized rabbits blood flow was recorded from the exclusively muscular masseteric artery in response to brief spontaneous contractions (BSC) of the masseter muscle, artery occlusion (AO), muscle compression (MC) and muscle stretch (MS). Activation of masseter muscle was monitored by electromyography (EMG). Responses to AO were also recorded from the mostly-cutaneous facial and the central ear arteries. Five animals were also tested in the awake condition. The hyperemic response to BSC (peak amplitude of 394 ± 82%; time-to-peak of 1.8 ± 0.8 s) developed with a latency of 300-400 ms from the beginning of the EMG burst and 200-300 ms from the contraction-induced transient flow reduction. This response was neither different from the response to AO (peak amplitude = 426 ± 158 %), MC and MS (p=0.23), nor from the BSC response in the awake condition. As compared to the masseteric artery, the response to AO was markedly smaller both in the facial (83 ± 18 %,) and in the central ear artery (68 ± 20 %) (p<0.01). In conclusion, the rapid contraction-induced hyperemia can be replicated by a variety of stimuli affecting transmural pressure in muscle blood vessels and is thus compatible with the Bayliss effect. This prominent mechano-sensitivity appears to be a characteristic of muscle and not cutaneous vascular beds.